SC: ActionInterpreter.execute might return nil to not create a next tx

[bchamagne]

Description

This PRs allows smart contracts to end without a "next_transaction".
I know @Neylix wants to go one step further (allow contract to have a next transaction even when it ends on a non-transaction-statements) but I believe it should be done in a new issue. (CREATED: #870)

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Tested with a smart contract that use this code:

actions triggered_by: interval, at: "0 * * * *" do	
	calls = get_calls() 
	if calls != [] do 
		[...]
		add_uco_transfers(transfers)
	end
end

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules